The
Tijuana Slough National Wildlife Refuge (NWR) is a coastal plain estuary
located in San Diego County, California. It represents one of the
few remaining relatively undisturbed tidally influenced wetlands in southern
California. Several rare and endangered species are located at the
estuary including the California least tern, the salt marsh bird's beak,
and the light-footed Clapper rail. The slough has been subject to
contamination from a variety of sources in the Tijuana River watershed
such as municipal/industrial discharges from Mexico, landfills, agriculture,
nonpoint source runoff, and illegal dumping in the United States and Mexico. The Sweetwater Marsh NWR was
established as a mitigation bank for several large water resource related
projects in San Diego County. It is located close to the Tijuana
Slough and encompasses the largest remaining wetland area in San Diego.
It has also been subject to contamination from a variety of sources including
storm water runoff and industrial/municipal sources. This study
took place from 1988 to 1992 and was aimed at monitoring the contaminant
levels in various animals, plants, and sediments of the area. The
contaminant concentrations accumulating in food items of many birds was
also monitored. The contaminant concentrations relative to threshold
values for specific contaminants were examined and the concentrations
between the two sites were compared.

Results
and discussion:
Most organic and inorganic compounds tested were found to be below threshold
levels in the various plants, fish, sediments, birds, and invertebrates
sampled. However, a few compounds were above threshold levels and
should be considered when evaluating the overall contamination of the
two sites. The
following inorganics were found to be above low effect thresholds (the
sediment concentration indicating a level of contamination that has the
potential to affect some sensitive benthic organisms) in the sediments
at Tijuana Slough NWR; cadmium, chromium, copper, mercury, lead, nickel,
and zinc. Only zinc exceeded the level above which adverse effects
are likely, so the impacts of these elements on the biota are not likely
to be major. The organic compound p,p'DDE was found to be above
NST ERL levels (the concentration below which adverse effects are seldom
expected) and was equal to the ERM (the sediment concentration above which
adverse effects are likely) concentration in 1989. These concentrations
could also have moderate levels of effects on biota. Based
on the concentrations found, minor to moderate impacts to benthic invertebrates
are possible. The only
organic to exceed the low effects threshold for Sweetwater NWR was p,p'DDE
in 1992. The organics
and inorganics in invertebrates at Tijuana Slough were all within the
known acceptable level. At Sweetwater NWR total PCB and copper
concentrations in invertebrates were above dietary thresholds for birds,
so additional monitoring is necessary to determine if the potential for
impact exists. The organics
and inorganics in plants collected at Tijuana Slough were also all within
acceptable bird dietary threshold levels. At
Sweetwater NWR copper was below levels toxic to plants in 1989, but above
levels in 1990. Impacts on plants due to copper toxicity include
induced iron chlorosis, thick roots, and the inability to put forth new
roots. There were no detectable organics in the plants collected
at Sweetwater NWR. The plant samples were also analyzed in consideration
of their being food resources for birds. Lead was below the acceptable
dietary threshold for birds in 1989, but above it in 1990.
In the fish at Tijuana Slough, zinc was found to be slightly elevated
above the normal level each year. This can have an impact on growth,
reproduction, and survival of the fish. Fish were also examined
as an important dietary resources for birds, and it was found that all
organics and inorganics were within acceptable threshold levels.
At Sweetwater Marsh NWR zinc was found to be above normal threshold levels
in all four years sampled.Only
birds from Tijuana Slough were collected and their liver tissues were
sampled for contaminants. Selenium was found to be above the level
identified for reproductive impairment in 1987 and 1988.

Summary:
It appears that Sweetwater Marsh NWR had higher levels of environmental
contaminants than did Tijuana Slough. The concentrations of most
organic compounds and inorganic elements appeared to be below biologically
significant threshold levels. However, long-term bioaccumulation
of contaminants can result in impacts and is a cause of concern.

Recommendation: Additional
sampling of trace metals at Sweetwater Marsh NWR
is recommended due to the elevated concentrations of these contaminants
that were found fairly consistently in the samples.Minimal
sampling at Tijuana Slough recommended to confirm the benefits of the
International Wastewater Treatment Plant and the South Bay Ocean Outfall
which take sewage out of the Tijuana River for treatment and offshore
discharge theoretically resulting in decreases in contaminant inputs to
the Tijuana Slough NWR.Ongoing
service participation in watershed management and pollution prevention
efforts should be made to facilitate the reduction of contaminant leads
into these two refuges and enhance the benefits of the IWTP and SBOO for
Tijuana Slough NWR by reducing nonpoint sources of contaminants.(return to top)